In the past, much interest has focused on methods for improving downhole placement of well treatment fluids used in acid stimulation and hydraulic fracturing operations. Acid simulation of a hydrocarbon producing formation, such as by matrix acidizing, enhances the production of hydrocarbons. In this procedure, acid or an acid-forming material is injected into the formation and the acid reacts with minerals in the formation. As a result, near-wellbore permeability is improved by the opening of channels or wormholes within the formation. In addition to dissolving formation materials, the acid may remove blockages caused by natural or man-made conditions. The procedure is especially prevalent in the treatment of carbonate formations since the reaction products are soluble in the spent acid.
Early attempts at optimizing the placement of acid downhole focused on injection of a simple acidic solution into the wellbore. Such attempts proved to be inefficient as the fluid often reacted or was spent too quickly. Such treatment fluids were therefore incapable of penetrating deep into the formation, thereby limiting their effectiveness to very near-wellbore applications. Thus, where the treated subterranean formation contained sections with varying permeability, the injected acid typically acidized the zone within the formation which had the highest permeability and the highest degree of water saturation. A permeability contrast between areas of high permeability (treated areas) within the formation and areas of low permeability (untreated areas) resulted.
It is necessary that acid placement downhole be optimized in order to provide uniform distribution of treatment fluid over the zone being treated. Chemical, as well as mechanical, methods have been developed in order to divert the flow of treatment fluids from the higher permeability and/or water saturated sections of the formation to the lower permeability or oil bearing sections. The difference between chemical and mechanical diversion is that chemical diverting agents achieve diversion by increasing flow resistance inside the created channels, whereas mechanical diversion controls the fluid entry point at the wellbore. Hence chemical diverting agents are often considered to be internal diverting agents compared to external mechanical diversion.
Chemical diversion has been previously achieved by the use of aliphatic polyesters, such as polylactic acids. While such products exhibit desirable degradation rates at higher temperatures, typically in excess of 250° F., they are extremely slow to dissolve at lower temperatures. As a result, such materials have not been useful in reservoirs having a bottomhole temperature lower than 250° F. Alternatives have therefore been sought for the use of such polyesters in reservoirs having bottomhole temperatures lower than 250° F.